Michael J. Fox and Christopher Lloyd reunited Saturday in the Big Apple during a 'Back to the Future' panel at New York Comic Con ... and when the two former costars met onstage, they greeted each other with a big hug, which got thunderous applause from the fans.

Check out the clip ... you see MJF out there by himself at first, taking a seat on the couch then he pops back up when Chris walks out, and wraps him up with open arms.

It's a super cool moment, and you can tell it meant a lot to both of them -- especially by the way Mike went in for the embrace. Afterward, they sat down and chopped it up ... reportedly going on to tell old stories about the trilogy, but particularly about Pt. 1 (the classic).

Michael reportedly said at one point ... "You guys have given me my whole life," adding a poignant nugget about his Parkinson's. Mike said, "The best thing that happened in my life was this thing. Parkinson's is a gift. I've said to people it's a gift and they say, 'You're nuts.' I say, 'Yeah, but it's the gift that keeps on taking.' But it's a gift and I wouldn't change it for anything ... It's not about what I have, it's about what I've been given."